Not from any real source but my own estimation, I’d say the worst guns for the environment are paintball guns. The fuel they use is straight-up bonafide carbon dioxide, and the average rate of fire for a paintball gun is much higher than other guns because more of the slow-speed rounds need to be fired to “walk” them onto your target.

Some other possibilities would be military tanks and ships, muzzle-loaders, and the kind that shoots the neutron bullet into the core of a nuclear bomb.

But, as cthomas said, the manufacture of all guns is harmful to the environment.